1. Field of the Invention
The invention concerns a band-pass filter written in an optical waveguide, which can be an optical fiber, a guide integrated into a semiconductor optical component or into the silica.
2. Description of the Prior Art
The formation of a filter in a waveguide is known in the prior art and consists in writing a Bragg grating into the guide. The prior art proposes various techniques for writing a grating of this kind into an optical fiber as disclosed in patents U.S. Pat. No. 4,474,427, U.S. Pat. No. 4,725,110, U.S. Pat. No. 5,104,209 and U.S. Pat. No. 5,367,588. Each technique is based on localized modification of the refractive index obtained by exposing the core of a fiber, typically doped with germanium, to spatially modulated ultraviolet light. An optical filter using a Bragg grating of this kind has a reflection spectrum with a peak centered on a wavelength called the Bragg wavelength. In transmission, a filter of this kind has a rejection band corresponding to the reflection peak.
The article "Long-period fiber gratings as band-rejection filters" by A. M. VENGSARKAR et al., published in OFC'95, PD4 (1995), describes a band-rejection optical filter in the form of a long-pitch Bragg grating (the pitch is in the order of one hundred .mu.m) written in an optical fiber from which the coating is locally removed. The pitch of the grating is chosen so that the fundamental mode guided in the core of the fiber is coupled at a given wavelength to a cladding mode which is then rapidly attenuated as it propagates in the cladding, because of losses at the cladding-coating interface.
French patent application n.degree. 95 14434 describes a band-pass optical filter written into a waveguide that has three Bragg grating portions and two intermediate areas. Each portion has a Bragg wavelength equal to the pass-band wavelength .lambda.p. The Bragg grating has a short pitch that produces contradirectional coupling between the go mode and the return mode of the fundamental mode. Each intermediate area creates a phase-shift of .pi. between these two propagation modes.
In some applications this filter does not procure a sufficiently wide rejection band on either side of the pass-band. Further, it has the drawback of necessitating an optical isolator on its upstream side to block propagation of the contradirectional mode.
French patent application n.degree. 96 02620 describes a band-pass filter formed by writing at least two long-pitch Bragg gratings into an optical fiber. The optical fiber is tapered to define two substantially adiabatic transition areas delimiting an intermediate area. A first long pitch Bragg grating is written in the intermediate area. A second long pitch Bragg grating is written in the transition area on the side receiving an optical signal. The effective index of the fiber in this area decreases along this transition area. The presence of the second grating in this area leads to coupling between the LP01 and LP02 modes over a large bandwidth and this results in rejection over a large bandwidth. The first grating produces codirectional coupling between the LP01 and LP02 modes at a wavelength which is a function of the pitch of the first grating. It produces an LP01 mode signal that conveys the power of the incoming optical signal for the wavelengths of coupling between the two modes by this first grating and an LP02 mode signal that is attenuated by leakage at the cladding-coating interface. This results in rejection over a very wide band, except for the band defined by the first grating. The combination of the two gratings therefore constitutes a band-pass filter.
The fabrication of this prior art filter has the drawback of necessitating the writing of two gratings instead of one.
The aim of the invention is to propose a band-pass filter that does not have the drawbacks of the prior art filters.